[Server] Serve account moderation through the database interface (#7849)

The moderator account commands still reached Servatrice_DatabaseInterface through
its concrete class: cmdResetUserPassword ran the privilege lookup inline and
called the concrete-only changeUserPasswordWithLegacyBackup, while the avatar,
banner and blacklisted-banner commands called concrete-only helpers. Promote
those four methods to Server_DatabaseInterface, move RemoveUserBannerResult into
the interface, and add getUserAdminFlags for the reset privilege lookup. The
escalation guard, audit records, had_banner response field and every response
code stay in the handler.

libcockatrice_network/libcockatrice/network/server/remote/server_database_interface.h

@@ -771,6 +771,12 @@ public:
     {
         return false;
     }
+    virtual bool changeUserPasswordWithLegacyBackup(const QString & /* user */,
+                                                    const QString & /* passwordVerifier */,
+                                                    const QString & /* legacyCredential */)
+    {
+        return false;
+    }
     virtual void setForcePasswordChange(const QString & /* user */, bool /* force */)
     {
     }
@@ -792,6 +798,31 @@ public:
     {
         return false;
     }
+    /** @brief Outcome of removing a user's banner, so a missing user stays distinguishable. */
+    enum class RemoveUserBannerResult
+    {
+        Removed,
+        NothingToRemove,
+        NoSuchUser,
+        DatabaseError
+    };
+    virtual RemoveUserBannerResult removeUserBanner(const QString & /* userName */)
+    {
+        return RemoveUserBannerResult::DatabaseError;
+    }
+    virtual bool removeUserAvatar(const QString & /* userName */)
+    {
+        return false;
+    }
+    virtual int clearBlacklistedBanners()
+    {
+        return -1;
+    }
+    /** @brief The raw admin bitmask of a user; false when the query failed or the user is missing. */
+    virtual bool getUserAdminFlags(const QString & /* userName */, int & /* adminFlags */)
+    {
+        return false;
+    }
     /** @brief Outcome of reading or writing one account field. */
     enum class UserFieldResult
     {